The present invention relates to an electronic circuit apparatus for sealing a multiwiring substrate with LSI and the like mounted thereon.
The objects for sealing the electronic circuit apparatus includes the follows:
(1) To cut off a LSI, wirings and the like on the multilayer interconnection substrate (multiwiring substrate for short) from the external atmosphere to prevent corrosion, and entry of foreign matter.
(2) To obtain an airtight seal, replacing the internal atmosphere with inert gas having a good thermal condcutivity, and transmit the generated heat from LSI to outside to cool chips.
It is a requirement that the sealing life of the seal is longer than the product life of the electronic circuit apparatus. FIG. 1 is a sectional view of a conventional electronic circuit apparatus having a sealing member with LSI mounted thereon. Reference numeral 1 designates a ceramic substrate, which is a multiwiring substrate formed with a wiring layer consisting of conductors such as W and Mo. Reference numeral 4 designates an LSI chip which is connected to a wiring of the ceramic substrate 1 by a method such as wire bond, tape carrier, CCB (Controlled-Collapse Bonding). Reference numeral 3 designates an input/ output pin connected to the ceramics substrate 1 by soldering. Reference numeral 5 designates a sealing metal cap, which is normally formed of 42% alloy (Fe-Ni 42% alloy), cobalt and the like. The cap 5 is adhered onto a metallized portion 2 applied to the periphery of the surface of the ceramic substrate 1 by a soldering material 6 of AuSn, silver solder or the like. Similar sealing members are discussed in NIKKEI ELECTRONICS, Mar. 26, 1984, page 178 to 184.
The aforementioned prior art has the following problems.
(1) The connection life of a sealing portion is determined by the consumption power of LSI, a heat release value determined by the number of LSIs mounted on the ceramics substrate, a material of the cap and the material of solder, and the size of the ceramics substrate. That is, when a temperature of the ceramics substrate rises from T.sub.1 to T.sub.2 due to the heat generation of LSI, an expansion by .DELTA.l (.alpha.l =.DELTA..alpha..multidot..DELTA.T.multidot.l) as shown in FIG. 2 occurs from an original length l due to the thermal expansion of the ceramics substrate. Because the material solder is softer than the 42% alloy used for the metal cap, a large stress and strain in the material of solder 6 is produced principally in the corner portion 7 of the cap 5. The stress and strain produced are repeated by the ON/OFF operation of LSI, and the sealing portion finally becomes cracked due to the thermal fatigue, losing its function. As the ceramics larger, substrate becomes the stress and strain with respect to the sealing portion also increases; thus a long life of the sealing portion cannot be guaranteed.
(2) Since flux (surface active agent) or the like is used when the cap and the ceramics substrate are connected by the solder, and when the shape of the connection portion hardly flat, the flux is escaped outside the solder, and as the flux is once introduced, a void 8 tends to occur within the solder as shown in FIG. 3. This void 8 reduces the thermal fatigue life of the sealing portion.